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Evaluation of Eradication Strategies for OJD
Ovine Johne’s disease (OJD) is caused by a host specific strain of Mycobacterium avium subspecies paratuberculosis (Mptb). This and the cattle strain of Mptb are spreading through domestic and non-domestic populations of ruminants around the world. OJD is considered to be a relatively new disease in Australia (25 – 50 years) that at present has infected less than 10% of sheep flocks nationally. The great majority of infected flocks are concentrated around a number of foci of infection in the southern mainland and adjacent islands. OJD is highly infectious but difficult to detect early in the cycle of infection because of a lengthy incubation period (2 – 3 years) and insensitive tests for subclinically infected sheep. It impacts on sheep enterprises primarily through increased mortality rather than subclinical production losses. Currently there is international debate among gastroenterologists and microbiologists regarding Mptb’s involvement in the aetiology of Crohne’s disease in humans. In 1998 steering committees representing the sheep industries and state and federal governments agreed on a national program of regulation, education and research to contain and improve our understanding of the epidemiology and pathology of OJD under Australian conditions. This report presents the results of a field evaluation of the biological efficacy and economic viability of destocking and restocking strategies to eradicate OJD from farms in south eastern Australia.
A total of 44 farms managing 45 monitor flocks were contracted to participate in the field evaluation. To facilitate decontamination, these farms were destocked of all susceptible species (primarily sheep and goats) for 15 to 21 months including two summer periods. The farms were restocked with at least 700 monitor sheep that were purchased from flocks complying with ovine Johne’s disease Market Assurance Program disease risk assurance standards. The flocks were monitored for evidence of OJD for the next three years via clinical investigations of ill-thrift and culture of samples of faeces collected from at least 500 monitor sheep two and three years after restocking. Mptb positive cultures were typed to distinguish between sheep and cattle strains of the bacteria.
The results in summary
• 28 of 41 flocks (68%) that were monitored to the completion of the project, presented with evidence of OJD following three years of exposure to decontaminated sites.
• 39 of 40 Mptb positive faecal cultures collected from these flocks were identified as sheep strain.
• There was a 20 fold reduction in the mean apparent prevalence between destocking (18%) and three years after restocking (0.9%) the 28 (re)infected sites.
• Eradication failures appear to have been primarily of local origin (ie reinfection from neighbouring flocks and/or incomplete decontamination)
• Efforts to identify infectious refugia in the summer prior to restocking yielded only one Mptb positive culture from 279 samples of topsoil, dam water and dam sediment
• 20 year simulations of net farm income based on data derived from farms participating in the field evaluation revealed that destocking and restocking was less profitable than vaccination as an OJD management option.
This page was last updated on 12/11/2014
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